1. Field of the Invention
The present invention relates to a technique for synthesizing a plurality of images into one image.
2. Related Background Art
Up to now, for example, as an x-ray sensing apparatus that creates an image from the amount of x-rays that have passed through the interior of an object to be sensed (the interior of a human body, etc.), there has been proposed a device in which a spatial distribution of the intensity of the x-rays that have passed through the object to be sensed is converted directly into an electric signal by a large-sized x-ray sensor panel, and the electric signal is converted into a digital value through analog-to-digital (A/D) conversion so that the x-ray image of the object to be sensed is available to image saving, image processing, image observation or the like as a digital image.
As the above-mentioned x-ray sensor panel, for example, in the case where the object to be sensed is a chest region of a human body, and the chest region of the human body is going to be sensed with x-rays all at once, there is employed a sensor panel that is about 40 cm×40 cm in size. The x-ray sensor panel of this size is brought substantially in contact with the chest region of the human body, and x-rays are irradiated onto the x-ray sensor panel from a direction facing the human body, to thereby obtain a digital image based on the intensity distribution of the x-rays that have passed through the chest region of the human body by one x-ray sensing.
Also, in the case where the fine structure of the human body is going to be sensed with x-rays, there is used an x-ray sensor panel having pixel resolution about 0.1 to 0.2 mm. In this case, the x-ray sensor panel becomes very large-sized to the degree of 2000×2000 pixels to 4000×4000 pixels.
As a method of reading image information from the above-mentioned large-sized x-ray sensor panel at high speed and with stability, there are proposed, for example, the following two methods.
(1) Relatively small-sized segment sensor panels are combined together in the form of tiles to constitute one large-sized sensor panel. The respective sensor panels are driven concurrently, and image information (electric signal) resultantly obtained is digitalized by an A/D converter.
(2) In order to collect the image information at high speed or to shorten substantial information wiring length on a sensor panel, a sheet of large-sized sensor panel is divided into sub-parts and driven, and image information (electric signal) resultantly obtained in each divided sub-part is digitalized by amplifiers and A/D converters which are disposed independently.
However, in the above-mentioned conventional x-ray sensing apparatuses, when the sensor panel is driven by not a single system but a plurality of systems independent from each other to collect the x-ray image information of the object to be sensed as in the above-mentioned manners (1) and (2), the characteristics of the amplifiers, the A/D converters, etc., which process the electric signals which are outputs of the respective segment panels, fluctuate independently (a change due to the environments, a change with a time and so on). This leads to the following problems.
For example, in the case where a single sensor panel is divided into four parts, and the respective segment panels are driven independently, when the x-rays that have passed through the object to be sensed are entered to the sensor panel, the electric signals that are outputted from the four segment panels of the sensor panel are digitalized by the amplifiers and the A/D converters which are independent from each other, and then temporarily saved as the four segment image data.
In this situation, the four segment panels of the sensor panel are driven under control in synchronization with a timing where the x-rays are irradiated to the object to be sensed.
Then, the same operation as the above-mentioned sensing operation is conducted under a condition where no x-rays are irradiated, thereby saving data of a fixed pattern such as offset.
In this situation, data related to a variation in the gains of the respective pixels in the four segment panels of the sensor panel is stored in the memory in advance. That information is generated by irradiating the x-rays toward the sensor panel in a state where there exists no object to be sensed, and acquiring the image information.
Then, the fixed pattern data saved in the memory is subtracted from the four segment image data acquired in advance, and further, correction for each of the pixels is conducted due to the gain variation data.
Here, the gain variation data for each of the pixels is obtained by irradiating the x-rays in a state where no object to be sensed exists. For example, in a normal medical spot, since it is very difficult to acquire the gain variation data for every x-ray sensing, the gain variation data is acquired by the x-ray sensing once a day.
Also, the fixed pattern data is acquired at a time that is very close to the x-ray sensing time, but not at the same time as the x-ray sensing time.
The above-mentioned time difference in the data acquirement may results in change of the environments (temperature, humidity or the like) where the data is acquired, and thus leads to the possibility that change may occur in the characteristic of the sensor panel, the amplifiers disposed for the four segments of the sensor panel, or the like.
For that reason, different characteristics appear for each of the segment images, and a definite boundary appears between the segment images.
Therefore, in order to solve the above-mentioned problem, there has been proposed in Japanese Patent Application Laid-open No. 2000-132663, for example, a structure in which a component having a characteristic continuous in a boundary direction in the vicinity of the boundary of the above-mentioned segment images is extracted, and the characteristic component is removed. According to this structure, in the case where the variation of the segment image is relatively small, it is very effective in solving the above problem, only the boundary portion can be smoothed, and as the result, correction is not required over the entire image.
However, in the above-mentioned structure, there is a case where the amount of correction becomes large, and a sense of incongruity over the image as a whole cannot be removed by only the partial correction. Also, in the case where the important image information exists along the boundary in the vicinity of the boundary of the segment images, there occurs such a problem that the image information is damaged.